Some key points:Different kinds of tests include:Unit tests: to check the correctness of several modules (classes).Customer's tests: functional, system and acceptance tests. All these tests check the behavior of the system as whole. In the XP methodology, these tests are written by the customer, given the program skeleton.Integration tests: to help test the interaction of several levels of the application.Developer tests: tests used by developers to verify the whole code, new pieces of code and/or new functions.JUnit is very useful when dealing with developer's test.